364 REPORTS ON THE STATE OF SCIENCE.—1912. 
example of this. The carbide separating from solid solution during 
cooling is at first in a state of ultramicroscopic division, and troostite 
has been described as a solid colloidal solution of carbide in iron.?% 
If more slowly cooled, finely granular sorbite and laminated pearlite 
are successively obtained. Pearlite has been commonly regarded as the 
normal form of the eutectoid, but it is found that further heating 
causes the lamine to contract, producing beaded forms,!°* and ulti- 
mately the carbide segregates into coarse masses,?°° so that the steel 
presents the paradoxical condition of containing both structurally free 
iron and carbide.1°* . 
The dispersion of a newly formed solid phase in an ultramicroscopic 
form through a crystalline mass of metal is not uncommon.?7 It is 
observed in alloys of nickel and iron, of cadmium and tin,}°* and also 
in the # solid solution of copper and zinc.1°® This last case is of 
special interest. The 6 solution is not stable below 475°, at which 
temperature it is resolved into a mixture of the a and y phases, which 
at first remain in a state of ultramicroscopic division. Segrega- 
tion is extremely slow, even at temperatures only slightly below the 
transformation point, but is accelerated by the presence of the a or 
the y phase in excess, or by the presence of a third metal in solid 
solution, such as aluminium. 
When crystals of a solid solution separate from a liquid, the com- 
position of the crystals is only uniform if the rate of cooling is so slow 
that time is allowed for continual readjustment of equilibrium between 
the solid and liquid phases by diffusion within the solid. If this is 
not the case, the crystals have a zonal structure, the outer zones being 
relatively richer than the inner in that component which lowers the 
freezing-point. Such zonal or ‘ cored’ structures are usually present 
in solid solutions of metals cooled under ordinary conditions. Although 
it has been attempted to explain these structures as produced by purely 
mechanical causes,” there is no doubt that the difference between 
adjoining zones is a chemical one, as is shown by the behaviour of the 
crystals on etching. It can be shown in an effective manner by 
immersing a specimen of a tin bronze, for example, in a solution of 
copper sulphate, when metallic copper is deposited only on those zones 
which are richest in tin.*"* 
When such an alloy is annealed at a temperature well below the 
melting-point diffusion takes place, and the composition of the crystals 
becomes uniform. This process is easily observed by means of the 
microscope. The cores in ana solid solution of tin in copper dis- 
appear when the alloy is heated to 750° for three hours,*” and the time 
required in other cases is very similar. 
108 C, Benedicks, Jour. Iron and Steel Inst., 1905, ii. 352 ; 1908, ii. 217. 
104 C, Benedicks, Metallurgie, 1909, 6, 567. 
105 J, E. Stead, Jour. Soc. Chem. Ind., 1903, 22, 340. 
106 H. F. Lange, Metallographist, 1903, 6, 9. 
107 ©. Benedicks, Zeitsch. Chem. Ind. Kolloide, 1910, 7, 290. 
108 W. Guertler, Intern. Zeitsch. Metallographie, 1912, 2, 172. 
10 H. C. H. Carpenter, Jour. Inst. Metals, 1912, '7, 123. 
0 F, Osmond and G. Cartaud, Compt. rend., 1904, 189, 404. 
Ml ¥, Giolitti, Gazzetta, 1908, 38, ii. 352. 
12 A, Portevin, Rev. de Métallurgie, 1909, 6, 813. 
